Okay, thank you for the pictures! Yes, he does have a touch of rot, but thankfully it's pretty minor and the Tetracycline will clear that up in a snap! As far as always being like that, the guy is wrong. Fish can heal and actually have amazing regenerative powers so once he kicks this infection, he'll be right as rain!
Something else you can try (I actually don't see any ammonia poisoning on him at all) is a Methylene Blue bath after the first round of antibiotics is all complete. It will help his rot if it's not gone by then but also help regenerate damaged and dead tissue to become new again! It helps oxygen through the bloodstream to the cells who are damaged. You use a bath tank of around a gallon if you have one and then 9 drops of M. Blue in the water. Stir it up and put your fish in the mixture and let him soak for 30 minutes. Careful with M. Blue, it stains! So you may have blue fingers for a little while lol. It will also stain silicone so try not to use it in a tank with silicone if you care about the tank. It's fine in acrylic tanks. After the bath of 30 mins are done, you net the fish out of the water and return him to his regular tank. You do not want any bath water going into the normal tank so this is why you net him and not cup him. Dump out old bathwater down the drain/toilet and do not reuse that water. You can do this nightly until he gets better.